1. Field of the Invention
The present invention is related to communication base station, and more particularly, to a base station system over a general IT platform, an adapter and base band processing unit.
2. Description of the Related Art
In recent years, wireless communication technology has developed rapidly. To achieve efficient processing of base station signal, Software defined radio (SDR) base station is proposed to perform signal processing.
FIG. 1A shows a structure diagram of a base station system over traditional SDR platform. As shown in FIG. 1A, the base station system includes a Radio Frequency (RF) unit 10 and a base band processing system 20. The RF unit 10 is used to transmit and receive RF signals to communicate with mobile terminals. Specifically, the RF unit 10 receives uplink data signal from a mobile terminal and transmits it to the base band processing system 20. It obtains processed downlink data from the base band processing system 20 and transmits it to the mobile terminal. The base band processing system 20 includes a RF interface 201 and a base band processing unit 202. The RF interface 201 is coupled to the RF unit 20 via optical fiber or hardware slot at one side, and is coupled to the base band processing unit 202 via an appropriate specific interface at the other side, so as to pass and transform data between the RF unit 10 and the base band processing unit 202. The base band processing unit 202 performs all the signal processing by using a specific hardware board. Specifically, the base band processing unit 202 includes a variety of specific hardware at a physical hardware layer, such as DSP, FPGA, embedded CPU etc. Simultaneous to the specific hardware, specific real time operating system is installed. Further, wireless protocol stacks in form of software are installed on the operating system and are used to perform processing of base band signals.
Since the SDR system shown in FIG. 1A performs signal processing using a specific hardware platform, for different communication standards and operating modes, different types and numbers of specific chips need to be used in most cases. Existing hardware platform needs to be altered when wireless standard is upgraded or changed. Thus, such systems have low flexibility and expansibility, but very high cost for developing, operating and maintaining hardware and software.
These problems existed in a traditional SDR base station. There is a proposed SDR system over a general IT platform. FIG. 1B shows a structure diagram of a base station system over a general IT platform. As shown in FIG. 1B, the base station system includes a Radio Frequency (RF) unit 10, an adapter 20 and a base band processing unit 30. The RF unit 10 of FIG. 1B is similar to that of FIG. 1A. The adapter 20 is used to perform data transmission and transformation between the RF unit 10 and the base band processing unit 30. In contrast to the RF interface of FIG. 1A, the adapter 20 is separated from the base band processing system as a standalone (hardware) component. The base band processing unit 30 performs signal processing via a general IT platform. Specifically, the base band processing unit 30 includes on physical hardware layer a general processor upon which a general operating system can be installed. Desired software framework and wireless protocol stacks can in turn be installed on the general operating system. Since the base band processing unit 30 is represented as a general IT computing platform, it can provide to outside general standard IT interfaces such as network interface, PCIe interface etc. Accordingly, the adapter 20 can be coupled to the base band processing unit 30 via these standard interfaces.
In the base station system shown in FIG. 1B, the base band processing unit 30 is implemented over a general IT platform. With the development of multi-core technology, the general IT platform is more likely to be implemented with multi-core chips, which makes computing capability of the IT platform grow rapidly. Since a general IT platform is utilized, it is possible to make a base station to be applicable to different wireless communication standards and operating modes via software modification without having to change physical components. Therefore, flexibility and expansibility of the base station is improved and development cost and operation cost is reduced. The base station system as shown in FIG. 1B still faces some technical challenges.
Those skilled in the art will appreciate that, either FDD (Frequency Division Duplex) or TDD (Time Division Duplex) mode can be utilized to communicate between a base station and a mobile terminal. Under FDD mode, the base station and the mobile terminal receive and transmit data on two separated symmetrical frequency channels, and receiving and transmitting channels are separated by different frequency bands. Under TDD mode, uplink and downlink communication between the base station and the mobile terminal use different time slots of a same frequency channel (that is, carrier wave), and receiving and transmitting channels are separated by time. Thus, the base station transmits signals to the mobile terminal at a certain time period, and the mobile terminal transmits signals to the base station at another time period. When the SDR system as shown in FIG. 1B is used in TDD mode, since receiving and transmitting channels are separated by time, time in the base station system is a very critical factor. Time synchronization in a traditional SDR system over specific hardware platform is not a problem, because in the system shown in FIG. 1A, for example, all the signals are processed by specific hardware and processing latency is fixed. All of the timing information in a base band processing system is provided by a GPS module. In this case, it is very easy to achieve time synchronization between a base band processing unit and a RF unit. In the SDR base station over a general IT platform shown in FIG. 1B, for example, only the adapter 20 is directly coupled to the GPS module that provides reference time, and the base band processing unit 30 can not directly obtain that reference time. The base band processing unit 30 over a general IT platform does not support the real time operation realized by specific hardware, instead, large buffers commonly used on general IT platform and task scheduling in general operating system etc often lead to unpredictable large round-trip delay. In addition, jitters will often occur to a system running on a general IT platform, leading to instability in time delay of signal processing. The factors described above make the base band processing unit 30 based on a general IT platform difficult to provide accurate time and can not reach time synchronization with the adapter 20.
To solve this problem, some solutions are set forth in prior art. In one solution, time to transmit downlink data is calculated by using time information of uplink data. However, in this solution, jitters of uplink data will accumulate to downlink, leading to large round-trip delay. Therefore, to make a SDR system over a general IT platform achieve better time synchronization used in TDD mode is a problem yet to be solved.
By modifying the installed software, the above SDR system over a general IT platform can be applicable to both TDD and FDD mode respectively. However, if the same SDR system can be made to adapt to the two operating modes simultaneously, that is, making an existing SDR system to be applicable to a TDD and FDD dual-mode radio access network, then operation cost can be further reduced. Thus, it is desirable to provide a base station system that is applicable to dual modes.